Torque Teno Virus (“TTV”), also referred to as transfusion-transmitted virus, is generally assigned to the Circoviridae family. It is generally recognized that TTV was first isolated from human transfusion patients (see for example, Nishizawa et al., Biochem. Biophys. Res. Comm. vol. 241, 1997, pp. 92-97). Subsequently, TTV or TTV-like viruses have been identified from other mammals, including swine, and numerous strains or isolates have been published (see for example, McKeown et al. Vet. Microbiol. vol. 104, 2004, pp 113-117).
Subsequent work as shown that TTV and TTV-like viruses are very common; however the pathogenesis of TTV, and the contributions it may make to other disease states (for example, those caused by other viruses and bacteria) remains unclear. For example, TTV infections appear to be common in humans, including even in healthy individuals, and such infections are often asymptomatic, and may remain for years. In addition, the general inability to propagate the virus in cell culture, and a lack of any clear mechanistic disease models, have made any overall characterization of TTV biology difficult. Notwithstanding that TTV viremia is elevated in human patients afflicted with other viral diseases, (such as hepatitis or HIV/AIDS), there is also considerable medical literature suggesting that TTVs are, in fact, avirulent, and await any clear actual association with known disease states. See, for example, Biagini et al., Vet. Microbiol. vol. 98, 2004, pp. 95-101.
In regard of swine, the situation is similar. There is considerable work suggesting that TTV infection is associated with, and contributes to, numerous diseases such as porcine circovirus disease (and its various clinical manifestations, such as postweaning multisystemic wasting syndrome and respiratory disease complicated by lung lesions), and PRRSV-associated disease (porcine respiratory and reproductive syndrome virus). See for example published international patent applications WO 2008/150275 and WO 2008/127279. Krakowka et al. also report on an often fatal disease in pigs referred to as PDNS (porcine dermatitis and neuropathy syndrome) which is described as a manifestation of disseminated intravascular coagulation, and for which combined infection by serotype 1 TTV and PRRSV virus was possibly implicated (Am. J. Vet Res, vol 69(12), 2008, pp. 1615-1622. PDNS disease was also correlated with porcine circovirus disease (notably PCV-2) and also with bacterial infections. Accordingly, while considerable work has been accomplished, there remains little work that definitively correlates porcine TTV infection with specific pathologies. Nonetheless, it has become reasonably clear that TTV infection can potentiate numerous disease states. Accordingly, there is a need for various classes of TTV reagents, such as high affinity antibodies, and for example, peptide fragments of TTV or whole virions that are highly immunizing, both to further our understanding of overall TTV biology and to vaccinate, directly or indirectly, against numerous disease states to which TTV may contribute.
Thus, although the possibility exists that TTV is the principle causative factor of diseases in swine, it seems more likely that numerous swine diseases either require the presence of more than one virus, or that the primary effect of certain “primary” pathogens is potentiated by TTV infection. As stated, the possibility exists that numerous diseases of swine can be treated or lessened by administering anti-TTV agents to affected or potentially affected animals. Notwithstanding the well established interest in TTV, effective vaccines have not emerged.
TTV is a small, non-enveloped virus comprised of negative polarity, single-strand circularized DNA. The genome includes three major open reading frames, ORF1, ORF2 and ORF3, which overlap, and ORF1 encodes the capsid protein. (Biagini et al., supra). For a detailed discussion thereof, please see the following references, which are incorporated by reference: Kakkola et al., Virology, vol. 382 (2008), pp. 182-189; Mushahwar et al., Proc. Natl. Acad. Sci, USA, vol 96, (1999) pp. 3177-3182; and T. Kekarainen and J. Segales, “Torque teno virus infection in the pig and its potential role as a model of human infection”, The Veterinary Journal, accepted Dec. 13, 2007 for 2008.
Despite the relatively simple genome, it has been generally very difficult to propagate the virus in cell culture or by other in vitro methods. The present invention is directed to recombinant constructs whereby TTV can be propagated in vitro, including via infectious clones. More particularly, the invention is directed to the discovery that effective vaccines can in fact be made from TTV, most particularly when the TTV antigen is the expression product of a single ORF, or a fragment thereof. In a preferred embodiment, the invention provides for ORF1 protein vaccines.